The invention is particularly applicable to optical fiber gyroscopes.
Several means, known per se, may be used to create broadband light sources. It is thus possible to use a fiber source doped with a rare earth, a super light emitting diode (SLED) or an edge light emitting diode (ELED). These light sources offer spatially single mode guide structures and therefore generating light with a strong spatial coherence due to an amplifier effect.
In the presence of an electrical or optical excitation, any one of these sources spontaneously emits photons in all directions. The applied excitation then entrains an amplification of the spontaneous emission in preferred directions by stimulated emission. This is called amplified spontaneous emission (ASE).
Unlike a laser, light emitted by these sources has low time coherence, in other words they are broadband sources and various means are used to preserve this property and avoid a laser effect.
Therefore one known technique is to use a fiber source 1 doped with a rare earth (particularly erbium) as an amplifying guide structure, as shown in FIG. 1. Fiber 1 with length L has a center line 10 with front end 2 and back end 3, and is symmetric about a transverse plane 4. The front end 2 and back end 3 are terminated by the front face 37 and back face 38 respectively.
A theory of the behavior of this amplifier guide structure assumed to be perfect is given below for information, in order to better identify the invention.
During operation, pump waves 6 and 9 with wavelengths .lambda.p are transmitted symmetrically towards fiber 1 by pump diodes and excite the rare earth. Once excited, the rare earth emits photons by spontaneous emission in all directions. However pump waves 6 and 9 amplify transmissions in the direction of the center line 10, thus creating two opposite preferred directions of propagation. This leads to two ASEs (Amplified Spontaneous Emissions): one 11 towards the front 2 of fiber 1 called the ASE+, and the other 12 towards the back 3 of fiber 1 called the ASE-, producing two light emissions 7 and 8 in front of and behind fiber 1.
Powers P1 and P2 are transmitted along the fiber 1 towards front 2 and towards back 3 respectively, as shown in FIG. 2, in which axis 15 defines the position along axis 10 in FIG. 1 and axis 16 gives the transmitted power.
Curve 13 shows the power P1 transmitted towards front 2. Curve 14 shows the variation of the power P2 transmitted towards back 3.
Emissions 7 and 8 obtained in outputs correspond to the two powers V3-V4 and V2-V1 respectively.
A major problem encountered in these amplification guide structures is that they have spectral instabilities of emitted light, usually due to thermal variations or the sensitivity of emitted light to excitation conditions. The instabilities may also originate from power variations of pump diodes or mode skips, for example.
It is often necessary to have precise control over the average emission wavelength .lambda..sub.m. This is particularly true when using optical fiber gyroscopes, this control being done with a precision of the order of 10.sup.-4 to 10.sup.-6. The stability of the average wavelength .lambda..sub.m is directly related to the stability of an essential characteristic of these gyroscopes, namely the scale factor, which must be precise.
One or several passive filters are usually used in order to successfully stabilize the emission spectrum S around a chosen average wavelength. This type of passive filter appears like a transmission window which is narrower than the emission spectrum window S. It is placed in an arbitrary position between the light source and the receiver. Its use leads to a stable emission spectrum S, and to very short wavelength fluctuations. However this passive filtering has the disadvantage of causing a severe loss of power.
EP-A-0,564,098 describes a device to create optical noise with a predetermined band width. This device comprises an optical amplifier producing an optical noise not polarized by spontaneous emission, a mirror reflecting optical noise towards the optical amplifier and a filter eliminating the optical noise components outside a predetermined band width.
EP-A-0,524,558 describes a super light emitting optical source. This source comprises an optical fiber doped with a rare earth, fiber pumping means to create a spontaneous emission, means selectively reflecting light on a given band width and optical coupling means at one end of the fiber with the reflecting means.
U.S. Pat. No. 5,283,686 concerns an optical system with a reflecting network. This optical system comprises a circulator, the first gate of which is coupled to an optical amplifier, a second gate to a Bragg network selectively reflecting the amplified signal over a given band width, and a third gate to an output line.